The coalition strike that Syria and Russia claim hit Syrian government troops may have struck a battalion of Syrian prisoners who were aligned with Syrian government forces but were in a separate location from the Syrian army, U.S. officials tell VOA.

The investigation into the strike is ongoing, but three officials say internal military reporting suggests the people hit in Saturday’s strike in Deir ez Zor province were not dressed in Syrian military attire and were using technicals — trucks modified with guns strapped on their backs — which are frequently used by Islamic State fighters and different from the tanks used by Syrian government fighters.

The officials say Syrian government prisoners may have been fighting for the regime as an alternative to captivity and appeared to be providing an “extra layer of protection” in an area outside the perimeter of the Syrian forces.

Speaking to VOA Monday, U.S. Central Command spokesman Col. John Thomas would not confirm or deny the details provided to VOA, saying only that the U.S. military was looking into several possibilities “to see if and how a mistake may have occurred.”

A brigadier general is being appointed by the U.S. military “today or very soon” to lead the investigation into the strike, a senior defense official said.

The U.S. military said Saturday the coalition believed it was targeting Islamic State fighters in Deir ez Zor province and halted the strike after Russian officials told the U.S. that vehicles and personnel being targeted were possibly part of the Syrian military.

Coalition members in the Combined Air Operations Center had earlier informed Russian counterparts of the upcoming strike, according to a U.S. military statement released Saturday.

The controversy comes as a U.S. and Russian-coordinated cessation of hostilities appeared to reduce violence in the country but failed to provide a sustained flow of relief supplies to civilians trapped in areas with sustained fighting.

The U.S. State Department reiterated in a statement on Monday that both steps were necessary before the U.S. and Russia would look into establishing a Joint Implementation Center to help with counter-Islamic State operations in Syria.

A U.S. official slammed the Russians’ lack of communication before the strike as a way to both “put the blame on the U.S. for the COH [cessation of hostilities] failure and take away from the Russians and Syrians’ humanitarian shortfalls in terms of getting aid to the Syrian people.”